The present invention relates to an elastically supported propeller installation or thruster for propulsion and/or steering of ships. More particularly it relates to a thruster of the type in which a propeller is operating in an open tunnel through the hull of the ship. Thrusters can be the source of a considerable noise and vibration level in a ship. Force pulses of a mechanical origin (bearings, gears and other transmission components) or of a hydrodynamic origin (inhomogeneous flow to propeller, cavitation etc.) occur in thrusters and tunnel tubes. These force pulses produce vibrations in the thruster and tunnel structures, and if the latter components are rigidly connected to the hull the vibrations will propagate through the hull structure and radiate in the form of acoustic noise in the interior fittings of the ship.
These structurally transplanted vibrations may cause high intensity noise over a wide frequency range. Typical noise levels in rooms adjacent to a conventional thruster installation are 80-90 dB(A).
If the design of the ship is such that people have to stay in this area, such noise levels will represent an unacceptable environmental strain on these people.
The object of the present invention is to provide a propeller thruster installation which is constructed in a manner to reduce as far as possible the vibrations transmitted from the propeller unit and tunnel to the hull of the ship, and to keep the noise in adjacent rooms at an acceptable level, while paying due attention to simplicity of installation and security of the ship.
A considerable noise and vibration dampening can be achieved by rigidly connecting the propeller unit with an inner tubular tunnel disposed coaxially within an outer tubular tunnel and secured to the latter by means of an elastic connection or isolator. This is a wellknown method for isolating machine vibrations from the surroundings. By adjusting the elastic properties of the isolator according to known rules a considerable amount of the oscillation energy excited in the thruster and the inner tunnel is prevented from spreading through the hull structure. However, this measure by itself is not sufficient to secure acceptable noise conditions in adjacent rooms. For additional noise-dampening it has been proposed to seal off the annulus between the inner, elastically mounted tubular tunnel and the outer, rigidly mounted tubular tunnel against the sea, and to confine a noise-dampening air volume in the annulus.
The dampening effect resulting from a such construction may reduce the noise level in adjacent rooms by the order of 15 dB(A).
However, a significant disadvantage in connection with this construction is that the seals required between the tubular tunnels for insulating the confined air volume, apart from necessarily being large, complex and therefore expensive, are located at positions where they are very vulnerable to damages and punctures.
Norwegian Pat. No. 135,462 and BRD Offenlegungsschrift No. 2,803,336 disclose thrusters of the above type, but in which the annulus between the tubular tunnels are filled with a sound-absorbing material that eliminates the need for a separate seal member to isolate the annulus from the sea. None of the above publications suggest any specific example of a such absorbing material. However, it is evident that if the material is to be capable of displacing the seawater in the annulus it must consist of a material having closed cells or pores, such as a rigid plastic foam or the like, since a soft plastic foam, i.e. a plastic foam with open pores, will absorb the water to a considerable degree rather than displacing it, thus eliminating the sound-dampening effect. This will also be the case if another type of soft sound-insulating material is used such as mats of mineral wool or the like.
The problem is however, that the use of rigid plastic foam or the like as a noise-absorbing, water-displacing material in the annulus fails to provide the desireable effect. This is because a such material, by virtue of being rigid, essentially eliminates the elasticity of the connection between the tubular tunnels, so that a large amount of the noise and vibrations generated by the thruster drive are transmitted to the hull structure. In practice, therefore, the solutions proposed in the two above mentioned publications are of little value.